The invention relates to vehicle tires comprising a tire carcass and side walls, wherein at least one layer of rubber forms a tire tread with a tread surface on top of the carcass. A tread pattern is provided in said tread rubber layer comprising a plurality of blocks at least in the circumferential direction of the tire and separated by grooves, and anti-skid stud holes within at least some of said blocks. Anti-skid studs are inserted in said stud holes, said studs comprising having a flat lamellar configuration with a stem portion and a basal portion. The diameter or the main and minor dimensions of said stud holes has/have a predetermined relationship to the respective dimensions of said studs. The invention further relates to vehicle tires of said type with a tire tread comprising at least two layers of rubber, wherein the base of the tread consists of a first harder rubber layer next to the tire carcass and the cap of the tread consists of a second more resilient rubber layer forming the tread surface, and elastic cushions disposed at the interior of the anti-skid stud holes.
There have lately been various attempts to enhance the grip of winter tires in particular. The grip can be increased with a pattern on the tread of the tire especially with a view to driving on snow. However, in driving on a snowy or icy roadway, the properties of the rubber forming the tread and the anti-skid studs fixed to the tread will have a more notable impact. The friction between the road surface covered with ice and/or compacted snow and the tread of the tire can be increased with the use of special friction rubber qualities in the tread. Such rubber qualities are very soft, i.e. they have high elasticity and a hardness typically in the range from 55 to 65 shxc2x0 A.
They are commonly called xe2x80x9ccap rubbersxe2x80x9d. If, however, the rubber layer on top of the tire carcass is throughout formed of such soft cap rubber alone all the way to the tread surface, the tire will sway too much during driving to be apt for many purposes of use, precisely due to this excessive elasticity. This is why the treads of tires have lately been formed of a combination of such soft cap rubber and a harder rubber layer as disclosed in publication US-4 603 721. The harder rubber quality, placed closer to the tire carcass, is commonly called xe2x80x9cbase rubberxe2x80x9d, and its hardness is typically in the range from 65 to 75 shxc2x0 A. The surface pattern being pressed into this kind of a rubber layer composite within a tire mold for vulcanization, the major part of the height of the blocks for the surface pattern, i.e. the vertical dimension of the grooves from the tread surface, will consist of the more elastic cap rubber, whereas the portion between the surface pattern and the carcass and a part of the height of the surface pattern will be chiefly of the harder base rubber. This design yields a xe2x80x9cfriction tirexe2x80x9d, having a high friction coefficient with regard to a hard driving surface, while the roadholding and driving properties of the tire still remains good owing to the harder base rubber layer. This kind of xe2x80x9cfriction tiresxe2x80x9d are expressly intended to be used and are used without any kind of additional anti-skid elements, in order to avoid wearing of roads.
The use of a harder base rubber layer between the surface pattern and the tire carcass, and partly within the surface pattern blocks, entails the same drawbacks as in the case of more conventional tires, in which the tire tread is entirely made of a harder rubber than the cap rubber mentioned above. When such a tire is provided with conventional anti-skid studs in the stud holes, the bottom flange of the stud within the tire tread opposite to the stud tip will be located in and against this hard rubber, and as a consequence of this, a relatively great force will be required on a road to press the tip of the stud to the plane of the tread. This will occur in any case under the weight of the vehicle, but the great penetrating force of the stud will entail considerable wear of the road surface and also considerable tire noise. What is more, this great penetrating force will prevent studs from being placed in the central parts of the tire, where the grip of the stud would otherwise be utilized at maximum. Current regulations explicitly forbid placing studs in the central parts of tires and allow studs to be placed close to the tire shoulder alone, where surface pressure between tire and road is lower, reducing wear of the road but also impairing the grip of the studs.
Publication JP-61-18506 discloses a tire with a supporting ring-band of a soft rubber quality extending in the circumferential direction of the tire and embedded in the tire tread between the carcass and outer parts of the tread. The width of this supporting ring-band is smaller than the total width of the tire and the supporting ring-band is positioned in the area of studs only. This construction of the tire tread destroys the at least the driving properties of the tire, making it to sway if the ring-band is made broad enough to allow a proper positioning of studs. If the supporting ring-band is kept so narrow that the acceptable driving properties are maintained the studs can be placed within so narrow an area that their effectiveness is considerably lowered.
Publication U.S. Pat. No. 3,831,655 discloses a special anti-skid spike for vehicle tires with at least one individual elastic member between the foot portion of the spike and the bottom of the hole in the tire tread. The individual elastic member may be prepared from a rubber material or a plastic material or it can be a metallic spring or a combination thereof. This disclosed elastic member is in each case fixed to the underside of the foot portion of the spike and preferably adhered either with glue or vulcanization thereto. The disclosed spike construction with an elastic member requires fabrication of tiny springy elements whether of rubber, plastic or metal, and an elaborate adhering of these elements to spikes, and/or a complicated insertion method to retain a proper mutual position of each of the springy elements and the respective spike bodies. The arrangement is impractical and the tires produced would be too expensive.
The well-known anti-skid studs are typically made up of a round pin of a cemented carbide, so called xe2x80x9chard-metalxe2x80x9d composed of mainly tungsten carbide as the hard component and cobalt in small amountsxe2x80x94generally from 5% to 10%xe2x80x94as the binder, placed in a larger round body of steel, having a stem portion towards the tread surface of the tire and a flange-like basal portion towards the carcass of the tire. The manufacturing is relatively complicated and expensive, because the steel body has to be machined, the cemented carbide pin has to be moldpressed and sintered, and finally these parts shall be soldered together. These anti-skid elements are pressed into holes which are preformed in the tire tread. They are known to have the disadvantages of a relatively large weightxe2x80x94density of WC is 15.7 g/cm3xe2x80x94and a tendency to increase their projection measured outwards from the tread surface during use, thus causing studs to become detached during the wear of the tire. The high stud weight and the large stud projection cause severe wear of the road surface and high tire noise. The tendency of the studs to become detached is also increased by the tilting of the stud, made possible by this known structure. Efforts have been made to reduce this tendency of the studs to become detached by using a stud design described in publication DE-28 04 939, in which the stud is V-shaped in cross-sectional planes parallel to the tread surface and which has, over the stud length, dovetail-like projections pointing outwards from the inside of the tire, but no actual basal flange. This design very effectively prevents the tilting of the stud and possibly thus, together with the dovetail projections, reduces the tendency of the studs to become detached. However, since studs of the type described have in the orientation of the tread the thickness of the conventional studs but are considerably longer than these, the weight of the studs described in the said publication is considerably greater than that of conventional round studs. This results in still higher tire noise, extensive wear of the road surface, and the subjection of the tire to high stresses which may damage it. Publication DE-1 027 089 discloses a non-slip element, which has a form of a thin corrugated band and which is vulcanized within the tire tread during manufacturing of the tire. In order to ensure the fitting between the rubber of the tread and the element, the element is provided with penetrating holes. This kind of suggested non-slip element is not practical nor effective, because it is not possible to produce elements with required dimensions and form using material, which has a hardness and strength high enough. If the production of a thin corrugated band of a material common in studs, like cemented carbides, and the installation thereof within the tread were somehow possible, the non-slip element would anyway break to pieces and is thrown out of the tire after a very short driving distance. Those materials which can be shaped to thin corrugated bands wear quite too rapidly to cause any discernibly added friction between the tire and the road surface, when compared to friction between a bare tire and the road surface. Thicker non-slip bands again lead to a too high weight, which damages the tire during driving.
The anti-skid studs described above work in the conventional manner that the grip of the stud is produced by the penetration of the xe2x80x9chard-metalxe2x80x9d part of the stud into the road surface under the effect of the projection of the stud tip from the plane of the tire tread. In this case the grip of the stud is thus based on the penetration force of the stud in the orientation of the tire radius. In the publications WO-96/28310 and EP-0 204 030 anti-skid elements based on another operating principle are described. In these publications, the anti-skid elements are positioned so that their projection from the tread surface is zero or very small, in which case, when the tire is rolling at a constant speed, there is either no contact or only very slight contact between the anti-skid element and the road surface. When the car is braked or accelerated, the deformation of the tire rubber causes the anti-skid elements to tilt, whereupon some edge of their very large end face oriented outwardly from the tire impinges on the road surface owing to the slight projection increase caused. In these publications, an effort has been made to maximize this projection increase by making the area of the stud end face very large in the orientation of the tread surface, because larger diameter causes greater projection during the tilting. For this reason, especially the stud according to publication WO-96/28310 is considerably heavier than a conventional anti-skid stud, as is admitted even in its Finnish priority application, wherefore for this later publication, an effort has been made to reduce this extra weight by giving the stem part of the anti-skid element the shape of a hollow cone. In spite of this, the weight of the anti-skid element according to publication WO-96/28310 is approximately double the maximum weight currently permitted by the regulations in Finland. Furthermore, experiments have shown that the projection, measured from the tread surface, of the studs according to these two publications increases considerably during use, causing rapid detaching of the anti-skid elements. In the main, the above also applies to the anti-skid element according to publication EP-0 204 030. The only difference in the anti-skid element according to this latter publication is the slightly flattened shape of the cross-section. This, again, causes additional problems, since the studs must be mounted in specific positions in holes in the tire tread, making the mounting considerably slower and more expensive.
Publication EP-0 864 449 discloses a passive lamellar non-slip element to be mounted in recesses preformed in vehicle tire tread pattern segments, the tire having a plurality of these segments, both in succession in the rolling direction and in the direction transverse thereto. The lamellar non-slip elements are composed of a strong and hard material and their shape comprising a stem portion towards the tire tread and a flange-like basal portion towards the tire body and an individual lamellar non-slip element being always within one pattern segment. The lamellar non-slip element is substantially lamelliform in shape, and the total thickness of the stem portion of the non-slip element in the orientation of the tire tread is at maximum approximately one-third, and preferably within the range of xc2xc-{fraction (1/20)}, of the width of the stem portion. The basal portion is wider than the stem portion and/or separated from the stem portion by a neck portion, the greatest thickness of the basal portion being at maximum five times the total thickness of the stem portion, and further the basal portion is substantially straight in the orientation of the width in order to provide tiltability of the lamellar non-slip element. The recesses, which are molded in the tire tread segments have substantially the same inner form as the outer configuration of the non-slip element allowing an effective tilting in a direction transverse to said width of the element. When the car is braked or accelerated, the deformation of the tire rubber causes the lamellar anti-skid elements to tilt, whereupon its gripping to the road is enhanced by the face of the element contacting the road surface. When the tire is rolling at a constant speed, there is either no contact or only very slight contact between the non-slip element and the road surface. This non-slip element is very lightweight, especially when compared to studs of WO-96/28310 and EP-0 204 030. The gripping of the non-slip element to the icy road is effective and the wear of the road surface is very low. As drawbacks are to be mentioned, that it has been noticed damages in the tire tread after a shorter or moderate mileage and/or under driving with a higher speed. Also these lamellar non-slip elements seem to broke or wear too rapidly after taking into use.
It is an object of the present invention to provide an anti-skid stud having a weight considerably smaller than the maximum weight permitted by the regulations. A second object of the invention is an anti-skid stud which would remain as well as possible attached to the tire tread also when the tire tread wears during use. A third object of the invention is to provide an anti-skid stud and a tire providing both good longitudinal grip and good transverse grip between the tire and the road, and at the same time causing as little abrasion of the road surface and as little tire noise as possible during driving. A fourth object of the invention is an anti-skid stud, which has a good endurance/dura-bility. The studs shall also have a compact structure, i.e. the anti-skid studs do not include elements movable relative to each other, and thus it is ensured that the studs will function also at the end of the life cycle of the tire and the stud. A sixth object of the invention is such a vehicle tire, in which the studs may be symmetric with regard to their central line, enabling the studs to be placed in the tire in any position. A seventh object of the invention is a vehicle tire to be fitted with studs providing a first-rate endurance/durability. A general object of the invention is an anti-skid stud and a tire in which these studs could be inserted so as to provide possibility to tailored driving and anti-skid properties. The manufacturing costs of the studs and the tire should be low, and further the mounting of the studs in holes preformed in the tread pattern blocks of the tire should be simple.
The problems described above can be solved and the objects defined above can be achieved by lamellar anti-skid studs and a tire provided with stud holes according to the invention. According to the first aspect of the invention the vehicle tire comprises a tire carcass and side walls; at least one layer of rubber forming a tire tread with a tread surface on top of the carcass; a tread pattern in said rubber layer comprising a plurality of blocks at least in the circumferential direction of the tire and separated by grooves; anti-skid stud holes within at least some of said blocks, said anti-skid stud holes having a generally circular cross-sectional form; lamellar anti-skid studs in said stud holes, said lamellar studs comprising a stem portion and a basal portion, which is wider than the stem portion and/or separated by a neck portion from the stem portion, the total thickness of said lamellar studs being at maximum one-third of the width of said lamellar studs in the orientation of said tread; and the diameter of said circular stud holes is substantially smaller than the width of said lamellar studs.
The most essential advantage of the invention is that the mass of the lamellar anti-skid stud according to it is considerably less than the maximum weights currently permitted for studs. At its best, the lamellar anti-skid stud according to the invention has a weight only approximately one-half of the maximum weight permitted by current official regulations, or possibly only ⅓-⅕ of the weight of the studs according to the reference publications described above. Another advantage of the invention is that the end face of the lamellar anti-skid stud remains, even during the wearing of the tire, close to its original position, i.e. close to the tread surface, in which case the remains well attached to the tire, and a continuously small rolling resistance lamellar anti-skid stud and low tire noise are obtained for the tire. The wear of the road surface will also remain very slight. A third advantage of the invention is that, when a lamellar anti-skid stud according to it is used, the longitudinal grip and transverse grip of the tire can be proportioned in a desired manner, and the grip of the tire will be good in different situations and on different road surfaces. A further advantage is that the flat lamellar anti-skid studs can be mounted in the circular stud holes in any position so that the width of the stud is oriented either transverse to the rolling direction of the tire or in line with the rolling direction or in any direction therebetween. All lamellar studs in a tire can have the same orientation or different orientations. A special advantage is that is not needed to produce different tires with stud holes for these alternative stud orientations, but the orientations of anti-skid studs are created when inserting the studs in the holes. So the longitudinal grip and transverse grip of the tire can be adjusted for different needs and purposes using the same basic tire and the same type of stud during mounting of studs. It is preferred that the layout or a selection of layouts, that is the orientation of the width of the studs in respect to rolling direction in different areas of the tread, of the studs is/are designed beforehand. The flat lamellar studs stay very firmly on these round stud holes during use and have no tendency to loosen or change their orientation.
According to a second aspect of the invention the vehicle tire comprises, instead of the circular stud holes as disclosed above, stud holes having a generally oval cross-sectional form with a main dimension and a minor dimension, which are transverse to each other and typically perpendicular to each other. The ration of the minor dimension to the main dimension is substantially smaller than the ration between the total thickness and the width of said lamellar studs. This alternative indeed limits the orientations possible for anti-skid studs, but gives protection against incorrect mounting of studs, and so provides a safety precaution. Because the stud holes also in this alternative have a generously rounded form, the tire with mounted studs have the same high endurance/durability as well as all the same other advantages as the embodiment mentioned above.
According to a third aspect of the invention the vehicle tire comprises a cushion disposed at interior end of said stud holes, that is the bottoms of the stud holes are provided with a cushion as described in an earlier patent U.S. Pat. No. 5 800 644. In no case shall the stud holes be so deep, that the bottom thereof reach the carcass of the tire, but there shall always be at least a thin layer of tread rubber forming the interior bottom. The underside of basal portion of the anti-skid studs generally bump at least partly against this cushion either initially or during driving when under pressure of the vehicle in the contact area between the tire and road surface only. Said cushions have a embossed form and may be composed of the one rubber quality of the tread, or preferably, in case the tread comprising a harder base rubber next to the carcass and a more resilient cap rubber on top of the base rubber, of the more resilient cap rubber. In this case the abrasion of the road and tire noise will be still lower than with the inventive tires with studs as described above, owing to the lower penetrating force of the anti-skid studs. The road abrasion and noise properties achieved by this type of tire having stud holes with cushions and fitted with lamellar anti-skid studs are approaching those of tires designed for summer time use, and nevertheless this inventive tire-stud-combination provides excellent grip with icy and or snowy road surfaces.
According to a fourth aspect of the invention the vehicle tire comprises oval or oblong stud holes and generally circular anti-skid studs inserted within said oval stud holes.
The further aspects of the invention are explained and become apparent from the detained description of the various embodiments of the anti-skid stud and various embodiments of stud holes as well as their combinations, as set forth later in this specification.